Linked compressor and expander (Lincompex) systems are well known in the telecommunications art. Such a digital Lincompex system is taught in U.S. Pat. No. 4,271,499 ('499) issued June 2, 1981, to Leveque, the Inventor of the present application. This Leveque patent '499 only teaches the transmitting of a voice signal over a Lincompex system. However, it has also been found that Lincompex systems can be utilized to transmit data having a complex waveform. Such a system that overcomes the inherent problem of data and envelope overlap (i.e., complex waveform) is fully disclosed in a co-pending application to Leveque, now U.S. Pat. No. 4,907,217, filed on Sept. 12, 1988, entitled "System and Method of Transmitting A Complex Waveform Over a Communication Channel Utilizing Lincompex Techniques".
In the voice transmission system, as disclosed in the Leveque U.S. Pat. No. '499, a voice signal is digitally transmitted using Lincompex techniques. This voice signal is band limited and exhibits an envelope 4 which is also band limited and does not overlap the voice band, (see FIG. 2(a)). An example of such a system is seen in FIG. 1 of the present application.
FIG. 1 of the present application illustrates a Lincompex system where voice information to be transmitted is introduced to an input 20 of the Lincompex system modulator as shown in FIG. 1(a). A control tone generator or envelope circuit 24 monitors the input voice signal. A compressor 22 compresses the introduced input voice signal. Compression is performed by dividing the signal by its envelope in pseudo-real time to produce a compressed voice signal. To develop the control tone, the system of FIG. 1(a) supplies the envelope signal developed from the output of the envelope detector 26 to a logarithmic (log) amplifier 28 which then develops a signal representative of the logarithm of the envelope signal. The output of this logarithmic amplifier 28 is supplied to a control terminal of a voltage control FM oscillator 30 which generates a frequency that varies about a center frequency F.sub.c in relation to the variation of the input voltage supplied to its control terminal from the logarithmic amplifier 28 to develop an envelope signal as an output of the control tone generator (envelope circuit) 24.
A summer 32 then sums the compressed voice signal developed at the output of the compressor 22 with the envelope signal developed at the output of the FM oscillator 30 to form a combined information signal.
In this speech transmission system, this combined information signal output from the summer 32 is provided to a transmitter 34 which transmits the signal over a desired transmission medium 36. In a typical embodiment, a single side band transmitter would normally transmit the modulated combined information signal across the airways in a known manner.
A conventional Lincompex demodulator is illustrated in FIG. 1(b). The demodulator receives the modulated combined information signal from the transmission medium 36, which normally includes an antenna for receiving radio waves from the atmosphere and supplying the received modulated combined information signal to a receiver 38 which demodulates the transmitted signal to reproduce the combined information signal. Typically, this receiver 38 would be a single side band receiver which mixes the received modulated combined information signal with the carrier frequency to reproduce at base band combined information signal. When transmitting voice, the combined information signal will exhibit the characteristics illustrated in FIG. 2(a).
To recover only the voice from such a combined information signal, a low pass filter 40 removes the envelope information 4 of FIG. 2(a) from the combine information signal to recover the compressed voice signal containing only the voice information 2. This compressed voice information is transmitted according to the Lincompex techniques at a substantially constant syllabic peak voltage which enables substantially complete modulation of the transmitter 34 of FIG. 1(a). This information must then be expanded to produce the necessary dynamic range for the recovered voice signal to be supplied at the output 60. Accordingly, an expander 42 is utilized which essentially multiplies the compressed voice signal developed at the output of low pass filter 40 by the envelope signal which is recovered by a control tone conversion circuit 44.
The control tone conversion circuit 44 includes a band pass filter 46 which recovers only the envelope signal 4 of FIG. 2(a) from the combined information signal. This frequency modulated envelope signal originally developed by the FM oscillator 30 of FIG. 1(a) is then frequency demodulated by a frequency demodulator or discriminator 48 to recover the logarithm of the envelope. An anti-logarithm amplifier 50 is then utilized to recover the original envelope developed by the envelope detector 26 of FIG. 1(a). This original envelope signal is then used to recover the original voice signal by expanding the compressed voice signal via the expander 42 to provide the original signal to the output 60.
As demonstrated by the above-described system, a voice signal can be transmitted and received digitally using Lincompex techniques. However, this system has not been able to compensate for frequency drift or the detuning of the transmitter/receiver system. It is necessary that the center frequency between the transmitter and receiver are synchronized to ensure that the control tone and voice signal are not distorted upon reception.
During transmission of the voice signal, frequency drift or the detuning of the transmitter/receiver system can cause a communication problem with the reproduction of the voice signal. More specifically, the control tone used in the reproduction of the compressed voice signal is very sensitive to a change in the frequency, frequency drift. If the frequency of the control tone drifts due to various factors such as detuning of the transmitter/receiver system, (frequency drift of transmitter or receiver modulation oscillators), the power in the reproduced voice becomes weaker in proportion to this frequency change. In the typical Lincompex System, the relationship of the attenuation in the reproduced voice signal and the change in frequency is governed by the ratio 1db/2hz. For example, a 30 hz change or drift in frequency can cause a 15db error or attenuation in the reproduced voice signal. To resolve this problem of power attenuation, a conventional Lincompex System as shown in FIG. 3 utilizes a frequency compensation circuit which corrects the frequency of the incoming control tone such that the attenuation problem is substantially eliminated.
As illustrated in FIG. 3, a conventional Lincompex demodulator includes a receiving unit 15 which receives a modulated combined information signal from the transmission medium 36, which normally includes an antenna for receiving radio waves from the atmosphere, which supplies the received modulated combined information signal to a receiver 38. The receiver 38 demodulates the transmitted signal to produce a demodulated version of the combined information signal. Typically, this receiver would be a single side band receiver which mixes the received modulated combined information signal with the carrier frequency to produce a baseband combined information signal.
To recover only the voice signal from such a combined voice/envelope information signal, a low pass filter 40 removes the envelope information 2 of FIG. 2(a) from the combined information signal to recover the compressed voice signal containing only the voice information 1. This compressed voice signal is transmitted according to the Lincompex techniques at a substantially constant syllabic peak voltage which enables substantially complete modulation of the transmitter 34 of FIG. 2(a). The compressed voice signal is passed through a fading regulator 58. The fading regulator 58 is a fading regulator similar to the one described in U.S. Pat. No. '499 which removes any audio level variations not removed by the automatic gain control circuitry in the receiver 38. This compressed modified voice signal output from the fading regulator is then expanded to produce the necessary dynamic range for the recovered voice signal to be supplied at the output 60. Accordingly, an expander 42, similar to the one described in U.S. Pat. No. '499, is utilized which essentially multiplies the compressed modified voice signal developed at the output of the fading regulator 58 by an envelope signal which is recovered by a control tone conversion circuit 27.
The control tone conversion circuit 27 comprises a band pass filter 46 which recovers only the control tone which is represented by the envelope information signal 2 FIG. 2(a) from the combined information signal. The control tone originally developed by the FM oscillator 30 of FIG. 1(a) is then passed through a frequency discriminator 48. The frequency discriminator 48 measures the instantaneous frequency of the control tone and produces a voltage level representative of this measured frequency. In this case, the voltage level also represents the logarithm of the original envelope signal. The anti-logarithm circuit 50 is utilized to recover the original envelope signal This logarithmic signal is then supplied to an anti-logarithm circuit 50. The anti-logarithm circuit 50 is utilized to recover the original envelope developed by the envelope detector 25 of FIG. 1(a). This original envelope signal is used by the expander 42 to recover the original voice signal by expanding the compressed voice signal to provide the original signal to the output 60. This control tone conversion circuit 27 is similar to the control tone conversion circuit described in the Leveque '499 patent.
The control tone conversion circuit 27 also comprises a calibration tone detector circuit 52 and a frequency measuring circuit 54. The calibration tone detector circuit 52 detects the initial calibration tone transmitted by the transmitter to determine when the frequency of the calibration tone is to be compared with a reference frequency to determine a frequency error.
This calibration tone is generated by a calibration tone generation circuit located in the transmitter in a manner similar to the process disclosed in the Leveque '499 patent. A typical calibration tone generator 88 of FIG. 1(a) includes an attenuator coupled to an output of a variable voltage divider. A switch 86 is also coupled to the digital compressor 22 and the calibration tone generator of a transmitter The attenuator and the switch are operated to momentarily allow an unattenuated and unmodulated calibration signal, which in the preferred embodiment corresponds to the frequency of the control tone signal at a center frequency to be transmitted prior to the compressed voice signal, thereby generating the calibration tone for a period of 200 to 300 milliseconds. This signal is generated in this example each time the microphone is keyed, or as desired.
The calibration tone detector circuit 52 generates a control signal representing that a calibration tone has been received. The detection of the calibration tone utilizes the conventional method described in the Leveque '499 U.S. Pat. The frequency measuring circuit 54 receives this signal from the calibration tone detector circuit 52 and compares this signal representative of the frequency of the calibration tone as generated by the discriminator 48 with the desired standard frequency, i.e., the intended control tone center frequency. If a difference in the frequencies is determined, the frequency measuring circuit 54 recognizes that a frequency error is present and generates a frequency error signal, a voltage in the preferred embodiment, which is stored and used by a frequency compensation circuit 56 so that the frequency compensation circuit 56 can correctly frequency translate the frequency of the control tone. The above frequency error determination can be implemented using software techniques.
An example of the frequency compensation circuit 56 is a typical phase shifting circuit or frequency translator as shown in FIG. 8(a). These circuits usually include a Hilbert Transform Circuit, an oscillator, phase shifter, multipliers, and a summer. In this example, (as shown in FIG. 8(a)), the signal path is split into two paths, A and B. Path A is connected to a multiplier 203. Connected to multiplier 203 is a voltage controlled oscillator 205 and a summer 211. The voltage controlled oscillator 205 is also connected to a 90.degree. phase shifting device 209. Furthermore, Path B is connected to a Hilbert Transform Circuit 201. Connected to the Hilbert Transform Circuit 201 is a multiplier 207. This multiplier 207 is connected to the 90.degree. phase shifting device 209 and the summer 211. The summer is connected to the input of the Lincompex demodulator.
An incoming signal is split into two paths, A and B. The signal traveling along path A is modulated, multiplied, by multiplier 203, with a cosine waveform type signal generated by the voltage controlled oscillator 205. The frequency of this cosine signal is determined according to the frequency error measured in the frequency measuring circuit 54 with a range between 0 to 100 Hz. The modulated signal from multiplier 203 is supplied to summer 211 to be added with a signal modified along path B.
The signal traveling along path B is first passed through a Hilbert Transform Circuit 201 which shifts the positive frequency components of the signal by -90.degree. and shifts the negative frequency components of the signal by +90.degree.. After being transformed by the Hilbert Transform Circuit 201, the signal is modulated, multiplied, by multiplier 207, with a sine waveform type signal. This sine signal is a 90.degree. shifted version of the cosine signal generated by the voltage controlled oscillator 205. The frequency of this sine signal is equal to the frequency of the cosine signal. The modulated signal is supplied to the summer 211 so that its negative and positive components can be summed with the modulated signal of path A. The signal generated by the summer 214 is a frequency compensated signal to be used by the Lincompex demodulator.
Another example of the frequency compensation circuit 56 is a typical frequency shifting circuit or frequency translator as shown in FIG. 8(b). These circuits usually include oscillators, multipliers, and filters. In this example, (as shown in FIG. 8(b)), a multiplier 103 receives the incoming signal. The multiplier 103 is connected to a first frequency compensator oscillator 101 and a first filter 105. The first filter 105 is connected to a multiplier 109. Connected to multiplier 109 is a second frequency compensator oscillator 107 and a second filter 111. The second frequency compensator oscillator 107 is connected to the frequency measuring circuit 54. The second filter 111 is also connected to the input of the Lincompex demodulator.
A first frequency compensator oscillator 101 produces a frequency corresponding to a first frequency which is greater than the bandwidth of the signal being shifted, necessary to prevent the problem of overlap of the two side band signals which occur when the signal is multiplied by a frequency less than the bandwidth of the signal. For example, if the bandwidth of the combination information signal is 3000 Hz, it might be desirable to set the frequency F.sub.OSCA of the first frequency compensation oscillator 101 at 9000 Hz. The output of the first frequency compensation oscillator 101 is mixed with the combination information signal within a first frequency compensation mixer 103 and the lower sideband is filtered by a frequency compensation highpass filter 105 which removes the lower sideband produced by mixing the F.sub.OSCA with the combined information signal using mixer 103 to develop a frequency raised combination information signal. A second frequency compensation oscillator produces a frequency F.sub.OSCB which corresponds to the first frequency minus the frequency change. For example, if the control tone must be raised 12 Hz, the second frequency compensation oscillator 107 generates a frequency F.sub.OSC2 =F.sub.OSC1 - 12, which is multiplied with the frequency raised combination information signal using a second frequency compensation mixer 109. This time a frequency compensation low pass filter 111 is used to remove the upper sideband, thereby raising (or lowering) the frequency of the combination information signal by the frequency error in this example, 12 Hz. In this conventional Lincompex System, the second frequency compensation oscillator 107 is a voltage controlled oscillator, thus producing a second frequency in accordance with the voltage level of the control signal received from the frequency measuring circuit 54.
Although the conventional Lincompex System can substantially eliminate the power variation in the reproduced voice signal due to frequency drift or detuning of the control tone, the conventional Lincompex System is not able to correct frequency drift of the modulated speech or data itself. Thus, the conventional system encounters at least two other problems due to frequency drift or detuning of the transmitter/receiver system. One such problem is commonly referred to as "duck-talk". This is caused by the voice signal being offset in frequency due to the frequency drift. The offset results in a distortion in the voice signal which creates a voice signal sounding like a duck, thus the name "duck-talk".
Another problem associated with frequency drift or the detuning of the transmitter/receiver is the distortion of the received data signals when data is transmitted. In a typical Data System to ensure proper reception of the data with the fewest number of errors, the filters used to obtain the data signal are as narrow as possible. This narrow banded aspect realizes, for a given transmission rate, fewer errors. However, if the frequency of the transmitted signal drifts or the transmitter/receiver system becomes detuned, a frequency error will occur, causing a portion of the data band signal to fall outside the narrow bandwidth of the receiving demodulator filters. In other words, the frequency error causes the data to becomes distorted, thereby destroying the quality of the data transmission.
While conventional Lincompex techniques allow the transmission of voice signals and can eliminate the power variation problem with respect to signal transmission, the conventional Lincompex Systems are not able to transmit a signal without the problem of "duck-talk" with respect to the voice signal and without data distortion due to frequency drift.